Touring Jupiter's Big Moons: Io, Ganymede, Europa, Callisto Touring Jupiter's Big Moons: Io, Ganymede, Europa, Callisto By SPACE.com Staff | November 22, 2010 10:41am ET Astrophysicists bring us up close with the large globes first glimpsed by Galileo as he observed Jupiter. Each is a very different world from its neighbors. credit : SPACE.com The Galilean Satellites : Io, Europa, Ganymede, and Callisto The Galilean Satellites : Io, Europa, Ganymede, and Callisto PIA01400, 1998-05-08 In this 'family portrait,' the four Galilean Satellites are shown to scale. These four largest moons of Jupiter shown in increasing distance from Jupiter are (left to right) Io, Europa, Ganymede, and Callisto. These global views show the side of volcanically active Io which always faces away from Jupiter, icy Europa, the Jupiter-facing side of Ganymede, and heavily cratered Callisto.

The appearances of these neighboring satellites are amazingly different even though they are relatively close to Jupiter (350,000 kilometers for Io; 1, 800,000 kilometers for Callisto). These images were acquired on several orbits at very low "phase" angles (the sun, spacecraft, moon angle) so that the sun is illuminating the Jovian moons from completely behind the spacecraft, in the same way a full moon is viewed from Earth. The colors have been enhanced to bring out subtle color variations of surface features. North is to the top of all the images which were taken by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft. Io, which is slightly larger than Earth's moon, is the most colorful of the Galilean satellites. Its surface is covered by deposits from actively erupting volcanoes, hundreds of lava flows, and volcanic vents which are visible as small dark spots. Several of these volcanoes are very hot; at least one reached a temperature of 2000 degrees Celsius (3600 degrees Fahrenheit) in the summer of 1997.

Prometheus, a volcano located slightly right of center on Io's image, was active during the Voyager flybys in 1979 and is still active as Galileo images were obtained. This global view was obtained in September 1996 when Galileo was 485,000 kilometers from Io; the finest details that can be discerned are about 10 km across. The bright, yellowish and white materials located at equatorial latitudes are believed to be composed of sulfur and sulfur dioxide. The polar caps are darker and covered by a redder material. Europa has a very different surface from its rocky neighbor, Io. Galileo images hint at the possibility of liquid water beneath the icy crust of this moon. The bright white and bluish parts of Europa's surface are composed almost completely of water ice. In contrast, the brownish mottled regions on the right side of the image may be covered by salts (such as hydrated magnesium-sulfate) and an unknown red component. The yellowish mottled terrain on the left side of the image is caused by some other, unknown contaminant.

This global view was obtained in June 1997 when Galileo was 1.25 million kilometers from Europa; the finest details that can be discerned are 25 kilometers across. Ganymede, larger than the planet Mercury, is the largest Jovian satellite. Its distinctive surface is characterized by patches of dark and light terrain. Bright frost is visible at the north and south poles. The very bright icy impact crater, Tros, is near the center of the image in a region known as Phrygia Sulcus. The dark area to the northwest of Tros is Perrine Regio; the dark terrain to the south and southeast is Nicholson Regio. Ganymede's surface is characterized by a high degree of crustal deformation. Much of the surface is covered by water ice, with a higher amount of rocky material in the darker areas. This global view was taken in September 1997 when Galileo was 1.68 million kilometers from Ganymede; the finest details that can be discerned are about 67 kilometers across. Callisto's dark surface is pocked by numerous bright impact craters.

The large Valhalla multi-ring structure (visible near the center of the image) has a diameter of about 4,000 kilometers, making it one of the largest impact features in the Solar System. Although many crater rims exhibit bright icy 'bedrock' material, a dark layer composed of hydrated minerals and organic components (tholins) is seen inside many craters and in other low lying areas. Evidence of tectonic and volcanic activity, seen on the other Galilean satellites, appears to be absent on Callisto. This global view was obtained in November 1997 when Galileo was 684,500 kilometers from Callisto; the finest details that can be discerned are about 27 kilometers across. The Jet Propulsion Laboratory, Pasadena, CA manages the Galileo mission for NASA's Office of Space Science, Washington, DC. JPL is an operating division of California Institute of Technology (Caltech). This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at URL .

Background information and educational context for the images can be found at URL Jupiter's Moon Callisto - Universe Today Universe Today by Matt Williams With 67 confirmed satellites, Jupiter has the largest system of moons in the Solar System. The greatest of these are the four major moons of Io, Europa , Ganymede and Callisto – otherwise known as the Galilean Moons . Named in honor of their founder, these moons are not only comparable in size to some planets (such as Mercury ), they are also some of the few places outside of Earth where liquid water exists, and perhaps even life. But it is Callisto, the fourth and farthest moon of Jupiter, that may be the most rewarding when it comes to scientific research. In addition to the possibility of a subsurface ocean, this moon is the only Galilean far enough outside of Jupiter’s powerful magnetosphere that it does not experience harmful levels of radiation. This, and the prospect of finding life, make Callisto a prime candidate for future exploration. Discovery and Naming: Along with Io, Europa and Ganymede, Callisto was discovered in January of 1610 by Galileo Galilei using a telescope of his own design .

Like all the Galilean Moons , it takes its name from one of Zeus’ lovers in classic Greek mythology. Callisto was a nymph (or the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis. The name was suggested by German astronomer Simon Marius, apparently at the behest of Johannes Kepler . However, Galileo initially refused to use them, and the moons named in his honor were designed as Jupiter I through IV, based on their proximity to their parent planet. Being the farthest planet from Jupiter, Callisto was known as Jupiter IV until the 20th century, by which time, the names suggested by Marius were adopted. The Galilean moons to scale, with Callisto in the bottom left corner. Credit: NASA/JPL Size, Mass and Orbit: With a mean radius of 2410.3 ± 1.5 km (0.378 Earths) and a mass of 1.0759 × 1023 kg (0.018 Earths), Callisto is the second largest Jupiter’s moons (after Ganymede) and the third largest satellite in the solar system.

Much like Ganymede, it is comparable in size to Mercury – being 99% as large – but due to its mixed composition, it has less than one-third of Mercury mass. Callisto orbits Jupiter at an average distance (semi-major axis) of 1,882,700 km. It has a very minor eccentricity (0.0074) and ranges in distance from 1,869,000 km at periapsis to 1,897,000 km at apoapsis. This distance, which is far greater than Ganymede’s, means that Callisto does not take part in the mean-motion resonance that Io, Europa and Ganymede do. Much like the other Galileans, Callisto’s rotation is synchronous with its orbit. This means that it takes the same amount of time (16.689 days) for Callisto to complete a single orbit of Jupiter and a single rotation on its axis. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing over the course of centuries due to solar and planetary gravitational perturbations. Size comparison of Earth, Moon and Callisto.

Credit: NASA/JPL/DLR/Gregory H. Revera Unlike the other Galileans, Callisto’s distant orbit means that it has never experienced much in the way of tidal-heating, which has had a profound impact on its internal structure and evolution. Its distance from Jupiter also means that the charged particles from Jupiter’s magnetosphere have had a very minor influence on its surface. Composition and Surface Features: The average density of Callisto, at 1.83 g/cm3, suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. Ice is believed to constitute 49-55% of the moon, with the rock component likely made up of chondrites, silicates and iron oxide. Callisto’s surface composition is thought to be similar to its composition as a whole, with water ice constituting 25-50% of its overall mass. High-resolution, near-infrared and UV spectra imaging have revealed the presence of various non-ice materials, such as magnesium and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds.

Model of Callisto’s internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior. Credit: NASA/JPL Beneath the surface is an icy lithosphere that is between 80-150 m thick. A salty ocean 50–200 km deep is believed to exist beneath this, thanks to the presence of radioactive elements and the possible existence of ammonia. Evidence of this ocean include Jupiter’s magnetic field, which shows no signs of penetrating Callisto’s surface. This suggests a layer of highly conductive fluid that is at least 10 km in depth. However, if this water contains ammonia, which is more likely, than it could be up to 250-300 km. Beneath this hypothetical ocean, Callisto’s interior appears to be composed of compressed rocks and ices, with the amount of rock increasing with depth. This means, in effect, that Callisto is only partially differentiated, with a small silicate core no larger than 600 km (and a density of 3.1-3.6 g/cm³) surrounded by a mix of ice and rock.

Spectral data has also indicated that Callisto’s surface is extremely heterogeneous at the small scale. Basically, the surface consists of small, bright patches of pure water ice, intermixed with patches of a rock–ice mixture, and extended dark areas made of a non-ice material. Compared to the other Galilean Moons, Callisto’s surface is quite dark, with a surface albedo of about 20%. Another difference is the nature of its asymmetric appearance. Whereas with the other Galileans, the leading hemisphere is lighter than the trailing one, with Callisto the opposite is true. Interior density structures created by an outer solar system late heavy bombardment onto Ganymede (top row) and Callisto (bottom row). Credit: SwRI An immediately obvious feature about Callisto’s surface is the ancient and heavily cratered nature of it. In fact, the surface is the most cratered in the Solar System and is almost entirely saturated by craters, with newer ones having formed over older ones. What’s more, impact craters and their associated structures are the only large features on the surface. There are no mountains, volcanoes or other endogenic tectonic features.

Callisto’s impact craters range in size from 0.1 km to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes, whereas those that measure 5–40 km usually have a central peak. Larger impact features, with diameters that range from 25–100 km have central pits instead of peaks. Those with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact. The largest impact features on Callisto’s surface are multi-ring basins, which probably originated as a result of post-impact concentric fracturing which took place over a patch of lithosphere that overlay a section of soft or liquid material (possibly a patch of the interior ocean). The largest of these are Valhalla and Asgard , whose central, bright regions measure 600 and 1600 km in diameter (respectively) with rings extending farther outwards. Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter.

Credit: NASA/JPL The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them – the older the surface, the denser the crater population. Based on theoretical considerations, the cratered plains are thought to be ~4.5 billion years old, dating back almost to the formation of the Solar System . The ages of multi-ring structures and impact craters depend on chosen background cratering rates, and are estimated by different researchers to vary between 1 and 4 billion years of age. Atmosphere: Callisto has a very tenuous atmosphere composed of carbon dioxide which has an estimated surface pressure of 7.5 × 10-¹² bar (0.75 micro Pascals) and a particle density of 4 × 108 cm-3. Because such a thin atmosphere would be lost in only about 4 days, it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto’s icy crust. While it has not been directly detected, it is believed that molecular oxygen exists in concentrations 10-100 times greater than CO².

This is evidenced by the high electron density of the planet’s ionosphere, which cannot be explained by the photoionization of carbon dioxide alone. However, condensed oxygen has been detected on the surface of Callisto, trapped within its icy crust. Habitability: Much like Europa and Ganymede, and Saturn’s moons of Enceladus , Mimas , Dione, Titan , the possible existence of a subsurface ocean on Callisto has led many scientists to speculate about the possibility of life. This is particularly likely if the interior ocean is made up of salt-water, since halophiles (which thrive in high salt concentrations) could live there. In addition, the possibility of extra-terrestrial microbial life has also been raised with respect to Callisto. However, the environmental conditions necessary for life to appear (which include the presence of sufficient heat due to tidal flexing) are more likely on Europa and Ganymede. The main difference is the lack of contact between the rocky material and the interior ocean, as well as the lower heat flux in Callisto’s interior. In essence, while Callisto possesses the necessary pre-biotic chemistry to host life, it lacks the necessary energy.

Because of this, the most likely candidate for the existence of extra-terrestrial life in Jupiter’s system of moons remains Europa. Exploration: The first exploration missions to Callisto were the Pioneer 10 and 11 spacecrafts, which conducted flybys of the Galilean moon in 1973 and 1974, respectively, But these missions provided little additional information beyond what had already learned through Earth-based observations. In contrast, the Voyager 1 and 2 spacecraft, which conducted flybys of the moon in 1979, managed to image more than half the surface and precisely measured Callisto’s temperature, mass and shape. New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter’s outermost large moon, Callisto, during its flyby in February 2007. Credit: NASA/JPL Further exploration took place between 1994 and 2003, when the Galileo spacecraft performed eight close flybys with Callisto. The orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters.

In 2000, while en route to Saturn, the Cassini spacecraft acquired high-quality infrared spectra of the Galilean satellites, including Callisto. In February–March 2007, while en route to Pluto, the New Horizons probe obtained new images and spectra of Callisto. Using its Linear Etalon Imaging Spectral Array (LEISA) instrument, the probe was able to reveal how lighting and viewing conditions affect infrared spectrum readings of its surface water ice. The next planned mission to the Jovian system is the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. Ostensibly geared towards exploring Europa and Ganymede, the mission profile also includes several close flybys of Callisto. Colonization: Compared to the other Galileans, Callisto presents numerous advantages as far as colonization is concerned. Much like the others, the moon has an abundant supply of water in the form of surface ice (but also possibly liquid water beneath the surface). But unlike the others, Callisto’s distance from Jupiter means that colonists would have far less to worry about in terms of radiation.

In 2003, NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto, for the purposes of investigating the possible existence of life forms embedded in the ice crust on this moon and on Europa. Artist’s impression of a base on the icy surface of Callisto. Credit: NASA The study proposed a possible surface base on Callisto where a crew could “teleoperate a Europa submarine and excavate Callisto surface samples near the impact site”. In addition, this base could extract water from Callisto’s ample supply of water ices to produce rocket propellant for further exploration of the Solar System. The advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate exploration on other Galilean Moons, and be an ideal location for a Jovian system way station, servicing spacecraft heading farther into the outer Solar System – which would likely take the form of craft using a gravity assist from a close flyby of Jupiter.

Reports filed by NASA’s Glenn Research Center and Langley Research Center – in December and February of 2003, respectively – both outlined possible manned missions to Callisto, as envisioned by HOPE. According to these reports, a mission that would likely involve a ship using a Mangetoplasmadynamic (MPD) or Nuclear-Electric Propulsion (NEP) drive system, and equipped to generate artificial gravity, could be mounted in the 2040s. So while Callisto may not be the best target in the search for extra-terrestrial life, it may be the most hospitable of Jupiter’s moons for human life. In either case, any future missions to Jupiter will likely include a stopovers to Callisto, with the intent of investigating both of these possibilities. We have many great articles on Callisto, Jupiter , and its system of moons here at Universe Today. Here’s one about how impacts effected Callisto’s interior , And here is one on all of the Galilean Moons . For more information, check out NASA’s Solar System Exploration page on Callisto . Astronomy Cast offers has a good episode on the subject, titled Episode 57: Jupiter’s Moons .

Share this: The Galilean Moons of Jupiter The Galilean Moons of Jupiter Summary: Jupiter has more than 60 known moons, but understanding the geology of its four largest will hopefully lead to some groundbreaking discoveries. Sections: The Galilean Moons Each of the Jovian planets has a number of moons, although Jupiter has the most with more than 60 catalogued to date. Jupiter's 4 largest moons exhibit some of the most interesting geology in the solar system. They were discovered by Galileo Galilei and are known as the Galilean moons. Jupiter's largest moon, Ganymede, is larger than Mercury while the other three are larger than Pluto. Outward from Jupiter: Io: Volcanoes and All Jupiter's Moon Io The Voyager spacecraft took the first close-up images of Io more than 300 years after the moon's discovery. The images showed a surface with no signs of craters from past impacts. What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress.

The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system. Io Close-Up (click to enlarge) Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red. Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes. Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics. The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Europa: What Lies Beneath? Europa Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old.

Europa's Interior Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle. Surrounding the rocky interior appears to be an icy layer 100 kilometers thick, the top few kilometers of which seem to be frozen solid. The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Analyzing Europa's Cracked Surface Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice. Other evidence comes from double-ridged cracks on the surface. Tidal flexing that allows water to well up and build ridges may create these cracks. Ganymede: Largest Moon in the Solar System Two Regions of Ganymede's Surface (click to enlarge) The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age.

The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions. If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa? Callisto: The Outermost Galilean Moon Callisto Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior. Callisto's Surface Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans.

Io (Moon) Facts Io size compared to the Moon and Earth Jupiter’s Galilean moons size comparison (Io, Europa, Ganymede and Callisto) Facts about Io Io has more than 400 active volcanoes on its surface. They make this little moon the most actively volcanic world in the solar system. The volcanism on Io is due to tidal heating, as the moon is stretched by Jupiter’s strong gravitational pull and by the lesser gravitational effects of the other satellites. The volcanoes of Io are constantly erupting, creating plumes that rise above the surface and lakes that cover vast areas of the landscape. Io has a very thin atmosphere that contains mostly sulfur dioxide (emitted from its volcanoes). Gases from the atmosphere escape to space at the rate of about a ton per second. Some of the material becomes part of a ring of charged particles around Jupiter called the Io plasma torus. The volcanic plumes of Io rise up as high as 200 km, showering the terrain with sulfur, sulfur dioxide particles, and rocky ash. Io has a number of mountains, some of which rise up as high as Mount Everest on Earth. The average height of Io’s peaks are around 6 km.

Io is made mostly of silicate rocks, and its surface is painted with sulfur particles from the volcanoes and frosts that are created as the atmospheric gases freeze out and fall to the ground. Robotic missions to Io could study its volcanism in closer detail. No human missions are planned as yet, due to the extreme radiation environment and highly toxic atmosphere and surface. Sources: A Quick Tour of Jupiter's Moons A Quick Tour of Jupiter's Moons A Quick Tour of Jupiter's Moons Jupiter and its moons as they might appear through a small telescope. Carolyn Collins Petersen Updated September 27, 2016. Meet the Moons of Jupiter The planet Jupiter is the largest world in the solar system. It has at least 67 known moons and a thin dusty ring. Its four largest moons are called the Galileans, after astronomer ​ Galileo Galilei, who discovered them in 1610. The individual moon names are Callisto, Europa, ​ Ganymede , and Io, and come from Greek mythology. Although astronomers studied them extensively from the ground, it wasn't until the first spacecraft explorations of the Jupiter system that we knew how strange these little worlds are.

The first spacecraft to image them were the Voyager probes in 1979. Since then, these four worlds have been explored by the Galileo, Cassini and New Horizons missions, which provided extremely good views of these little moons. The Hubble Space Telescope has also studied and imaged Jupiter and the Galileans many times. The Juno mission to Jupiter, which arrived in summer 2016, will provide more images of these tiny worlds as it orbits around the giant planet taking images and data. continue reading below our video Test Your General Science Knowledge Explore the Galileans Io is the closest moon to Jupiter and, at 2,263 miles across, is the second smallest of the Galilean satellites. It is often called the “Pizza Moon” because its colorful surface looks like a pizza pie. Planetary scientists found out it was a volcanic world in 1979 when the Voyager 1 and 2 spacecraft flew by and captured the first up-close images. Io has more than 400 volcanoes that spew out sulfur and sulfur dioxide across the surface, to give it that colorful look. Because these volcanoes are constantly repaving Io, planetary scientists say that its surface is "geologically young".

Europa is the smallest of the Galilean moons . It measures only 1,972 miles across and is made mostly of rock. Europa’s surface is a thick layer of ice, and underneath it, there may be a salty ocean of water about 60 miles deep. Occasionally Europa sends plumes of water out into fountains that tower more than 100 miles above the surface. Those plumes have been seen in data sent back by Hubble Space Telescope. Europa is often mentioned as a place that could be habitable for some forms of life. It has an energy source, as well as organic material that could aid in the formation of life, plus plenty of water. Whether it is or not remains an open question. Astronomers have long talked about sending missions to Europa to search for evidence of life. Ganymede is the largest moon in the solar system, measuring 3,273 miles across. It’s made mostly of rock and has a layer of salt water more than 120 miles below the cratered and crusty surface.

Ganymede’s landscape is divided between two types of landforms: very old cratered regions which are dark-colored, and younger areas containing grooves and ridges. Planetary scientists found a very thin atmosphere on Ganymede, and it’s the only moon known so far that has its own magnetic field. Callisto is the third-largest moon in the solar system and, at 2,995 miles in diameter, is nearly the same size as the planet Mercury (which is just over 3,031 miles across). It’s the most distant of the four Galilean moons. Callisto’s surface tells us that it was bombarded throughout its history. Its 60-mile thick surface is covered with craters. That suggests the icy crust is very old and hasn’t been resurfaced through ice volcanism. There may be a subsurface water ocean on Callisto, but conditions for life to arise there are less favorable than for neighboring Europa. Finding Jupiter's Moon's From Your Back Yard Whenever Jupiter is visible in the nighttime sky, try to find the Galilean moons. Jupiter itself is quite bright, and its moons will look like tiny dots on either side of it.

Under good dark skies, they can be seen through a pair of binoculars. A good backyard-type telescope will give a better view, and for the avid stargazer, a larger telescope will show the moons AND features in Jupiter’s colorful clouds. Hubble snaps rare images of three moons crossing Jupiter - LA Times Hubble snaps rare images of three moons crossing Jupiter Jupiter and five moons NASA, ESA and Z. Levay (STScI/AURA) These annotated images, taken by the Hubble Space Telescope, show Jupiter with five of its moons: Io, Callisto, Europa, Amalthea and Thebe. These annotated images, taken by the Hubble Space Telescope, show Jupiter with five of its moons: Io, Callisto, Europa, Amalthea and Thebe. (NASA, ESA and Z. Levay (STScI/AURA)) Amina Khan Contact Reporter Hubble captures a rare group shot of three of Jupiter's largest moons Here's a photo-op pretty enough to tempt any astronomer. With a series of well-timed shots, NASA’s Hubble Space Telescope has managed to catch three of Jupiter’s biggest moons whizzing across its surface in the same frame.

This Jovian lunar gathering, which took place Jan. 24, is a relatively rare event -- it happens only once or twice a decade. Hubble had to snap a series of images in fairly quick succession to capture the brief moment when all three could be seen hovering over the colorfully banded planet. The three moons, Europa, Callisto and Io, are three of the four Galilean satellites -- so named because astronomer Galileo Galilei discovered them in 1610. (The largest, Ganymede, was off-screen and too far to be considered part of the conjunction.) These four moons are Jupiter’s largest, and among the largest bodies in the solar system. To put that in perspective: these four moons are bigger than all the dwarf planets, Pluto and Ceres included. Three of them (Ganymede, Callisto and Io) are all larger than Earth's moon, and Ganymede is larger than the planet Mercury. In this image, the Jovian moons are traveling from the bottom left toward the top right as they cross Jupiter’s midriff, and their shadows are cast far in front of them; near the beginning of conjunction, Callisto’s shadow looks like it’s almost directly beneath Io.

A rare triple-moon transit of Jupiter, as viewed by the Hubble Space Telescope. (NASA, ESA and the Hubble Heritage Team (STScI/AURA)) The Hubble images also showcase in a single frame how different each of these moons are. Io, the leading moon at the top right, is an orange-yellow world, riddled with active volcanoes . Callisto, second in line, is ancient, cratered and brownish in color. Europa , the trailing moon in the bottom left, is a yellow-white, reflecting the ice covering its frozen exterior. (Europa has come to the forefront recently as one of the "water-worlds" in our solar system that may hide a subsurface ocean, and hold the potential for life.) Callisto: Facts about Jupiter's Moon, Callisto • The Planets Interactive Solar System Model Callisto Facts Callisto is the eighth moon of Jupiter and the second largest of the planet’s known satellites. It is the third largest moon in the solar system and is similar in size to the planet Mercury .

It orbits just beyond Jupiter’s main radiation belt and is the outermost of the Galilean moons – the four largest moons of Jupiter: Io , Europa, Ganymede and Callisto – discovered by Italian scientist Galileo in 1610. Size of Callisto compared to Jupiter Side by side comparison of the size of Jupiter vs it’s moon Callisto Facts about Callisto The name of the moon comes from Greek mythology. Callisto was a nymph who had an affair with the king of the gods, Zeus. His wife Hera then changed her into a bear before Zeus transformed her into a star and placed her in the sky as the constellation Ursa Major. This constellation is also known as the Great Bear, or the Plough. There is also a Little Bear constellation which was the son of Zeus and Callisto. All of the moons of Jupiter are named for Greek mythological figures seduced by Zeus.

Callisto is 99% the diameter of Mercury but only 1/3 the mass At a diameter of 4,820.60 km, Callisto is only 58.40 km less than the planet Mercury, but the mass of the moon is only around a third of Mercury’s mass – 107,593,737,963,819 billion kg compared to 330,104,000,000,000 billion kg for Mercury. The Galilean moons were discovered in 1610 by Galileo The four largest moons of Jupiter are known as the Galilean moons after Galileo, who discovered them in 1610. They weren’t discovered prior to this because they cannot be seen with the naked eye and Galileo was the first person to actually design a telescope to be used to look at the sky and our vast universe. Callisto isn’t a planet because it does not orbit the Sun Since Callisto is larger than the dwarf planet Pluto and a similar size to Mercury you might think it could be considered a planet.

However, it orbits the planet Jupiter and not the Sun , which is one of the primary factors in bodies being officially named as a planet (or dwarf planet ). There are more craters on Callisto than any other object in the solar system The surface of Callisto is the most heavily cratered and also one of the oldest landscapes in our solar system. Scientists believe it is a dead world with no signs of geologic activity like plate tectonics or volcanism on its surface. The moon is made of around 60% rock/iron and 40% ice and may contain liquid water. Callisto is composed of both rock and ice, in approximately equal measures, and there are also traces of water ice, carbon dioxide, silicates, and organic compounds on the surface. The Galileo spacecraft also revealed that the moon may have subsurface ocean of liquid water 100 km or more below the surface. Through a telescope, Callisto is brighter than our Moon Despite being 628,300,000 km from Earth , Callisto appears much brighter in the sky than our Moon when viewed through a telescope – despite our Moon being only 384,400 km.

This is because the surface of Callisto is made up of a very thick layer of ice which reflects the Sun much more than the surface of our Moon. Callisto is not affected by the gravity of other moons of Jupiter Unlike Io, Europa and Ganymede, Callisto doesn’t suffer from an orbital gravitational pull. This means there is less of a tidal heating – a build up of friction, orbital and rotational energy that are dissipated as heat in the crust of the moons and planets involved – which have caused the melting of surface ice from the rocky mantle of the Europa moon, for example. The reason it is less affected by Jupiter’s magnetosphere is that it orbits further away from the planet. Callisto is over a million miles from Jupiter. Earth’s Moon orbits around 238,855 miles from us, however Callisto orbits Jupiter at an average distance of 1,169,856 miles (1,882,700 kilometres).

One orbit of Jupiter takes Callisto 16.7 days It takes Callisto 16.7 earth days to orbit Jupiter and during that time it will have travelled more than seven million miles – 7,345,927 miles (11,829,191 kilometres) to be exact. Callisto moves at 18,400 miles per hour The moon has an orbital speed of around 18,400 miles per hour (29,530 kilometres per hour). Callisto is tidally locked to Jupiter. Similar to Earth and its Moon, Callisto is tidally locked to Jupiter. This means that the same face of Callisto is always pointing toward Jupiter and Jupiter would appear to be still when viewed in Callisto’s sky. The largest crater on Callisto is nearly 4,000 km across. The largest impact craters on Callisto are huge multi-ring basins which are surrounded by concentric rings that appear to be cracks that have been smoothed out by the slow movement of the ice over a long period of time.

Valhalla is the largest of these impact craters and stretches to around 3,800 km in diameter from the outer trough zone. It is named for Valhalla, the Norse God Odin’s hall where warriors are taken after death. Callisto has a very thin atmosphere. The atmosphere of Callisto is very tenuous and is composed mostly of carbon dioxide and probably molecular oxygen as well. Callisto may have formed from the gas and dust that surrounds Jupiter Unlike Earth’s Moon which may have been a piece of Earth that broke away after an impact billions of years ago, Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after the planet was formed 4.5 billion years ago. Callisto may be able to harbor life. Although it is not considered to be geologically active, the likely presence of an ocean beneath the surface of Callisto leaves open the possibility that it could harbor life. Also, it has low levels of radiation and has long been considered the most suitable place for a human base for future exploration of the Jovian system. However, it is though that the conditions may be more favorable on the nearby satellite, Europa.

Callisto Profile Rare Photos: Jupiter's Triple-Moon Conjunction in Pictures Rare Photos: Jupiter's Triple-Moon Conjunction in Pictures By SPACE.com Staff | February 5, 2015 03:00pm ET MORE Rare Sight on Jupiter Credit: Efrain Morales Rivera On Jan. 24, 2015, the shadows of three large moons of Jupiter crossed the face of the giant planet. The celestial shadow dance was captured by NASA's Hubble Space Telescope and amateur astronomers. See photos of the rare event in this gallery. HERE: Astrophotographer Efrain Morales Rivera of Aguadilla, Puerto Rico, sent in a composite photo showing the rare triple shadow transit of Jupiter's moons, Io, Callisto and Europa. Hubble Scopes Out Three Moons Transiting Jupiter Credit: ESA/NASA NASA used Hubble telescope observations of the triple-moon conjunction to create a short video of the event. Watch the Video Here . Hubble View of Jupiter Moon Triple Transit Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) Three moons and their shadows move across Jupiter during a rare triple transit on Jan. 24, 2015.

Europa is at lower left, Callisto is above and to the right of Europa and Io is approaching Jupiter's eastern limb. Europa's shadow is toward the left side of the image and Callisto's shadow to the right. [Read full story .] Hubble Space Telescope Photographs Rare Triple-Moon Conjunction Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) NASA's Hubble Space Telescope captured the rare sight of three of Jupiter's largest moons, Europa, Callisto, and Io, crossing the face of the gas-giant planet on Jan. 24, 2015. [Read full story .] Polar View of Jupiter System Credit: NASA, ESA, and G. Bacon, L. Frattare, Z. Levay, and F. Summers (STScI/AURA) A still image from an animation shows the positions of Jupiter's four largest moons during a rare triple-moon conjunction on Jan. 24, 2015. [Read full story .] Oblique View of Jupiter System Credit: NASA, ESA, and G. Bacon, L. Frattare, Z. Levay, and F.

Summers (STScI/AURA) A still image from an animation shows the positions of three of Jupiter's four largest moons during a rare triple-moon conjunction on Jan. 24, 2015. [Read full story .] Jupiter Triple-Moon Conjunction Diagram Credit: ASA, ESA, and G. Bacon, L. Frattare, Z. Levay, and F. Summers (STScI/AURA) A still image from an animation shows the positions and shadows of Jupiter's four largest moons during a rare triple-moon conjunction on Jan. 24, 2015. [Read full story .] Jupiter Moon Transit of January 24, 2015 Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) Three moons and their shadows cross Jupiter at 06:28 UT on Jan. 24, 2015. [Read full story .] Start of the Jupiter Moon Transit on Jan.

24, 2015 Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) Three moons and their shadows cross the face of Jupiter at the start of triple transit at 06:28 UT on Jan. 24, 2015. From left to right, Callisto and Io float above Jupiter's cloud tops. The shadows from Europa (not visible), Callisto and Io appear from left to right. [Read full story .] Jupiter's Moons Amalthea and Thebe Credit: NASA, ESA, and Z. Levay (STScI/AURA) While observing the Jovian triple-moon conjunction on Jan. 24, 2015, Hubble Space Telescope also captured the smaller moons Amalthea and Thebe wtith their shadows crossing the clouded face of Jupiter. [Read full story .] End of the Jupiter Moon Transit on Jan. 24, 2015 Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) Three moons and their shadows cross the face of Jupiter near the end of the triple-moon conjunction event at 07:10 UT on Jan.

24, 2015, as shown in this annotated image. [Read full story .] previous | next

Question: Io, Callisto and Europa are moons of which planet?
Answer: Jupiter